
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
105
“Transmit GFC Nibble-field” serial input port
Most of these functional blocks will be discussed in
some detail below. The Transmit Cell Processor will
read in ATM Cell Data from the TxFIFO. The first four
bytes of each cell is loaded into the “HEC Byte calcu-
lator”. The fifth byte of each cell will be read-in and
compared against a pre-defined “Data Path Integrity
Check” pattern. While this “check” is being performed;
the “HEC Byte Calculator” will take these first four
bytes of the cell, and compute a HEC byte value. This
HEC byte value will be written (or inserted) into the
5th octet position of the cell. Consequently, the “Data
Path Integrity Check” pattern will now be overwritten.
Bytes 6 through 53 (the cell payload) of each cell, are
sent onto the “Cell Scrambler” and are summarily
“scrambled”. Afterwards, the cell is reassembled (with
the first four header bytes, the newly computed HEC
byte and scrambled payload), and is routed to the
Transmit PLCP Processor or Transmit DS3 Framer.
When a complete cell is not available in the TxFIFO, a
cell is created by the “Idle Cell Generator”. The user
has the option of specifying the contents of the header
and payload of these Idle Cells via the μP-accessible
registers. The payload of the Idle Cell will be pro-
grammed with a repeating pattern of a byte contained
within an on-chip register. From this point on, the Idle
Cell is processed in the same manner as is an as-
signed (e.g., user or OAM) cell. A valid HEC byte is
computed over the four bytes of the programmed idle
cell header and is inserted into the fifth octet position.
The user has the option to disable the HEC Byte
Calculation and Insertion features for Idle cells, and
the contents of the fifth-header byte programmed reg-
ister may be transmitted directly.
The Transmit Cell Processor provides a means to trans-
mit pre-programmed OAM cells upon demand. The
content of this OAM cell is stored in an on-chip RAM
location, which will be referred to as the “Transmit
OAM Cell Buffer”. When the local μP decides to
transmit the OAM cell to the “Far-End” Terminal, it
writes a “1” to a certain register bit. The Transmit Cell
F
IGURE
16. F
UNCTIONAL
B
LOCK
D
IAGRAM
OF
THE
T
RANSMIT
C
ELL
P
ROCESSOR
B
LOCK
DataBusL[7:0]
DataBusH[7:0]
ReadB
WriteB
CSB
Configuration
and Status
Registers
TUSoC
TCelPresent
TFDat
CellOf52
TxGFC
TxGFCClk
TxGFCMSB
SendOAM
TDPChkPat
ICHECCalcEn
HECInsEn
HECErrMask
OAMSent
TDPIntegFail
Controller
CosetIn
GFCInsEn
ICGRegSel[5:0]
HEC
Calculator
HECEn
FIFOrlCDAT[7:0]
GFC[3:0]
HECSoC
TICCount
TCellCount
TxCelTxed
TCelRdClk
From Framer/PLCP
Scrambler
Idle Cell
Generator
OAM
Processor
TCelData[7:0]
ScramblerEn
TxCPInt
To Interrupt
Block
HECDat[7:0]
HeaderLoc
OAMCycle
OAMDataL[7:0]
OAMDataH[7:0]
ICDat[7:0]
From TxUtopia
To/From Pins
TxCPRegSel
TFIFORdENB
TFIFORCLK
H_PL